1. Field of the Invention
The present invention relates to a method for preloading a gear to be disengaged in a multi-step transmission of a motor vehicle and to a shift drum for a multi-step transmission of a motor vehicle, having at least one circumferential track, in particular a circumferential groove into which at least one axially displaceable track follower can engage, the circumferential track having flanks on which the track follower is guided.
The present invention relates, furthermore, to a shift drum-controlled multi-step transmission for motor vehicles and to a method for preloading a gear of a multi-step transmission to be disengaged.
2. Description of the Related Art
Such a shift drum and such a method are known from DE 199 24 335 A1.
In automatically shifting multi-step transmissions for motor vehicles, a distinction is made between two fundamental concepts. One concept is a combination of load planetary gear units and hydrodynamic converters. Particularly because of the hydrodynamic converter, the efficiency of such transmissions is not especially favourable. The second concept, which has increasingly been adopted in recent times, is based on an arrangement of a conventional manually-shifted transmission and of a conventional starting and separating clutch in the form of a friction clutch. Instead of these elements being actuated by the driver by means of a clutch pedal and shift lever, corresponding actuators are provided, which, in order to change gears, are activated in a mutually coordinated way as a function of an superordinated shift strategy.
In the manually-shifted transmissions designed as multi-step transmissions of the transfer box gearing type, when the gear is changed there is a collapse in traction force between the time point of disengaging a gear and the time point of engaging a new gear, since the friction clutch assigned to the multi-step transmission is opened in this case. The engine is, in this case, decoupled from the transmission and therefore also from the driving wheels.
In automated multi-step transmissions, the gear change itself is caused, for example, by means of one or more shift drums. These shift drums are suitable for sequential actuation of shift transmissions.
The shift drums are driven by means of an actuating drive, such as, for example, a motor, in particular an electric motor. For example, a pin-shaped groove follower, which is also designated as a shift pin, is guided in at least one guide or control groove of the rotatable shift drum. The shift pin is mounted axially displaceable with respect to the axis of rotation of the shift drum and connected to an actuator (shift fork, synchronizing unit, shift sleeves and the like) of the assigned transmission. If, then, a gear which is coupled to the pin is to be shifted, the shift drum and therefore also the control groove are rotated with the aid of the electric motor. The control groove has curved regions, also called shift teeth or shift cams, by means of which the shift pin is offset axially. In these curved regions, the control groove, normally running straight in the circumferential direction, is offset in the axial direction.
A plurality of shift pins assigned to various gears can be guided in the control groove. A single shift pin may, if appropriate, even be assigned to a plurality of gears, namely in such a way that an axial displacement of the shift pin out of a neutral position into a first axial direction engages a gear and an axial displacement of the pin out of the neutral position into the opposite axial direction engages another gear.
In the prior art, various proposals have been made as to how the predetermined sequential shift sequence can be avoided in a shift drum of this type. Thus, it is known per se to bring the shift pin out of engagement with the control groove by means of a radial movement of the control groove, i.e. perpendicularly with respect to the axis of the shift drum. This allows a shift tooth of the shift drum to move past under the engagement end of the shift pin, without causing an axial displacement of the shift pin.
In order to jump gears during a shift operation, it is known, furthermore, to provide shift grooves being widened in portions and to mount the shift drum, overall, so as to be axially displaceable such that shift teeth can be moved past the shift pin, without actuating the shift pin axially. Instead of the shift groove being widened only in portions, the shift groove may also be widened, overall, in the axial direction, in which case the groove width should be at least equal to the pin width plus the shift width, such that by axially displacing the shift drum by a distance corresponding to the shift width, the shift tooth or a plurality of shift teeth can be shifted effectively or shifted ineffectively.
If the width of the control groove significantly exceeds the width of the groove follower or of the shift pin, the groove follower (shift pin), at least in regions, is not guided in the control groove in both axial directions.
Instead of a widened control groove, DE 101 28 854 A1 proposes what is known as a bypass control groove, in which the shift pin can be moved past the shift tooth, without the shift pin being adjusted in the axial direction. That is to say, the control groove is simply continued rectilinearly, the control tooth constituting a branch with respect to the rectilinear control groove.
In order to influence the path which the shift pin has to take, a switch is provided. The switch is a guide part displaceable in the axial direction. In a first axial position, the switch steers a shift pin into the track of the shift tooth. In the second axial position different from the first shift position, the switch steers the shift pin into the bypass track, so that it is possible, in a sequential shift sequence, to jump a gear assigned to the switch. The axial displaceability of the switch thus serves for jumping gears in a sequential order.
In DE 195 43 645 A1, the shift drum itself is displaced axially, in order to jump gears in a sequential order.
DE 198 45 604 C1 discloses a method for disengaging a gear of a multi-step transmission.
Therein, a drive train likewise comprises a friction clutch and a multi-step transmission. The multi-step transmission transfers the power of an engine of a motor vehicle from a transmission input shaft to a transmission output shaft. The multi-step transmission is designed in a way known per se as a countershaft transmission and possesses a countershaft. The transmission comprises a plurality of wheel sets. Between the wheel sets, the transmission output shaft has arranged thereon a shift clutch formed with a synchronizing device, for the form-closed engagement and disengagement of the gears.
When a gear is being engaged, the synchronizing device is capable of reducing differential rotational speeds between the transmission output shaft and the gearwheel to be connected to the latter to zero and therefore of synchronizing the two transmission components.
In response to a gear-change requirement signal, which is triggered in a manual-shift mode, for example, by the actuation of a shift lever, but, in an automatic mode, is triggered independently by a central control, a lowering of the engine power first takes place, for example by gradually closing a throttle valve or regulating the injection quantity correspondingly.
Somewhat temporal offset and otherwise in parallel therewith, the clutch is actuated, so that the friction clutch is moved gradually from a completely closed position towards a completely opened position. During this movement, the friction clutch passes through what is known as a slip point. Up to the slip point, the friction clutch does not yet slip, and the rotational speeds of the engine and transmission shafts are identical. Beyond the slip point, the friction clutch begins to slip. A rotational-speed difference between the engine shaft and transmission input shaft occurs.
Furthermore, after the detection of the gear-change requirement signal, an activation signal for preloading the engaged gear is generated by the transmission control. The activation signal gradually rises and remains at a final value up to a time point at which the shift clutch is moved towards the open position.
Due to the gradual rise of the activation signal, the force acting on the shift clutch by means of the transmission actuator is gradually increased in the manner of a pretension.
It is to be noted that the force necessary for moving the shift clutch from the closed position into the open position depends on how high the load is which is transferred in the shift position or on how high the corresponding torque is. As long as a torque is transferred from the transmission input shaft, the shift clutch is “braced” circumferentially in the shift position on account of the positive engagement at the synchronizing device. This positive engagement may be enhanced by what is known as an undercut and by a draw-in effect which thus comes into action.
Due to the preloading of the shift clutch of the gear to be disengaged, it may happen that, because of the engine torque decrease introduced, the torque transferred from the transmission input shaft, although not yet being zero, has nevertheless reached a value such that the size of the pretension (force) exerted on the synchronizing device by the transmission actuator is correspondingly sufficient to move the shift clutch into the open position. The time of an interruption in traction force can be shortened thereby.
Systems or transmissions of this type with traction force assistance (for example, DE 199 08 602 A1), generally require a decoupling of the disengagement of the source gear and engagement of the target gear. In shift drum-actuated transmissions, this is generally possible only when two or more shift drums are provided.
The problem, on which the invention is based, is to specify an improved shift drum or an improved shift drum-actuated transmission and an improved shifting method (with preloading of the source gear), whereinin particular the control of preloading is facilitated.